VAV FAN COILS COMPETE WITH CHILLED BEAMS
18/09/2009
Fan coils have seen chilled beams increase market share, particularly multi service chilled beams MSCB, generally at their expense. This has been an evolving situation over the past few years and currently the market value of both options is about the same at around £25M. Because the unit cost of fan coils compared to MSCB is considerably less, the volume in terms of numbers of FCU is much greater. This therefore makes it still the most popular terminal air conditioning system but to hold this position many improvements to the basic fan coil system are required.
The most important issue for any system comparison is the energy consumption and carbon emissions.
Part L 2006 sets a target of 28% reduction in carbon emissions from the base in Part L 2002. But we all know it doesn’t stop there, in fact industry representatives are confirming the new 2010 Part L will target a further 25% reduction in carbon emissions with a view to getting commercial buildings carbon neutral by 2019.
With MSCB the most significant factor is the elevated chilled water temperatures 14ºC/17ºC compared to a conventional fan coil unit of 6ºC/12ºC. These higher chilled water temperatures can give MSCB significant periods of free cooling with associated lower carbon emissions. There is a further important benefit with these higher temperatures since it makes the use of ground water heat pumps a more effective and viable solution. This challenge to the energy credentials of fan coils systems can be met by using elevated chilled water temperatures of, say, 11ºC/15ºC. Although not as high as MSCB they do eliminate the use of condensate pumps and pipework.
When looking for energy savings sometimes the obvious targets can be overlooked as in the case of fan coils which run at 100% full speed irrespective of whether they are heating, cooling, or, as is generally the case, recirculating room air mixed with fresh air. With a conventional fan coil the airflow is constant at 100% whilst the heating or cooling coil valve is adjusted according to the room requirements. If the air volume was varied the saving in energy follows a cube relationship. That is to say if you reduced the air volume from 100% to 80% the power consumed would be .8 x .8 x .8 = .512 or 51.2%. In other words for a small reduction of 20% (100% to 80%) in air flow the energy from the motor could be reduced by nearly a half. If the air volume was reduced from 100% to 60% the power consumption would be reduced to a fifth.
The argument here is that the fan coil need only be operating at its 100% air volume when either maximum heating or cooling are required. At all other times the air volume can be reduced with significant energy and carbon emission savings.
By using VAV fan coils you can save something like 3.5W/m² and that could represent as much as a 8% saving on emissions from the building. The Part L2 saving target is, of course, higher than this, 28%better than the 2002 levels, but these savings do not come about all at once; they are inevitably made up of a whole series of savings which incrementally add up to or exceed the target.
As previously mentioned only a small speed reduction of 20% can cut in half the energy consumption from the fan motor. On a building of, say, 10,000m² with 400 fan coils this could equate to an energy saving of £15,000 to £20,000 per annum. That makes VAV fan coils an important energy saving development. However, it comes at a price – if they vary the air volume, designers must be extra careful to select diffusers that match. The speed variability of VAV fan coils itself makes the selection of the grilles and diffusers more critical because they need to work at a range of volumes.
If you vary the volume and go below a certain airflow limit you run the risk of air coming down off the ceiling because it doesn’t sustain enough energy to remain up there. This is called “dumping”.
Dumping can have serious consequences for the comfort in a space and that is why it is necessary to have specialist air distribution advice and probably fit anti-dumping devices to any critical diffuser selections.
Another factor that has been popular with MSCB is the inherent prefabrication element where all the system components are pre-assembled into a single unit. With fan coil systems this is also possible by pre-assembling and fitting the controls package including controllers, transformers, relays and temperature sensors. Flexible hoses, isolating valves and commissioning valves can also be fitted at the factory. With the use of EC motors it is possible to completely commission the fan coil system so that any on site activity is reduced to “checking” rather than “setting “.
These factors are assisting the fan coil market to stabilise and to maintain its market share.
